The field of the invention is AC motor drives for variable speed control of AC induction motors, and more particularly, high performance AC motor drives using pulse width modulation (PWM)techniques.
A motor drive for an AC induction motor includes a power section and a logic and control section. The power section receives power from a 3-phase AC source operating at 60 Hz frequency. The AC power is converted to DC power to provide a PWM inverter with a DC source for synthesizing voltages of different frequencies which are necessary to control the speed of an AC motor.
In high performance drives such as that described in U.S. Pat. No. 5,032,771, vector control, or field-oriented control is employed to control the speed and torque of the AC motor by developing command voltages for the PWM inverter. Such high performance drives employ motor stator current feedback which is resolved into a torque-producing or q-axis component of current, I.sub.q, and a flux-producing or d-axis component of current, I.sub.d. A motor speed feedback signal is also used for controlling the current components I.sub.q and I.sub.d to thereby control motor torque, speed and slip in accordance with the desired control strategy.
The operation of a closed loop, high performance drive is determined in part by the gain of the loops formed by the feedback from the motor. These closed loops include the PWM inverter, and their gains are, therefore, subject to variations in the gain of the PWM inverter at different motor speeds.
As disclosed in U.S. Pat. No. 5,121,043, the PWM inverter has three distinct regions of operation. At low motor speeds the inverter operates in a linear pulse width modulation mode in which the inverter chops the DC bus voltage into pulses that produce a sinusoidal current in the motor armature that is related to the input voltage command by a constant gain factor and the motor's impedance. Similarly, at high motor speeds the inverter changes to a six step inverter mode in which it produces a current, the fundamental component of which is related to the command voltage by a different gain factor and the motor's impedance. However, in the transition range between these two modes of operation the inverter gain is not constant. This transition range is referred to as the "pulse dropping region" since some of the triangular carrier pulses applied to the PWM inverter are not large enough to produce even a short voltage pulse or are nonexistent at the inverter output.
In this pulse dropping region, the inverter gain drops significantly and adversely affects the closed loop operation of the drive. For example, if a transient load is applied to a motor operating in this pulse dropping region, the drive will not respond properly to the speed or current feedback signal to correct the situation. This problem may be manifested as drops in motor speed or torque or as nuisance trips of the drive protection circuits due to current overload.